The Impact of Mediterranean SSTs on the Sahelian Rainfall ...
Large-scale transient variations of tropical deep convection forced with zonally symmetric SSTs
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Transcript of Large-scale transient variations of tropical deep convection forced with zonally symmetric SSTs
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Large-scale transient variations of tropical deep convection forced with
zonally symmetric SSTs
Zhiming Kuang
Dept. Earth and Planetary Sciences and School of Engineering and Applied Sciences
Harvard University
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Zonally symmetric SST cases can be interesting
SPCAM experiment modeled after earlier experiments by Marat Khairoutdinov
Log(Power)
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Zonally symmetric double-ITCZ
A simple model of convectively coupled waves
Kuang, 2008; Andersen and Kuang, 2008
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A moisture-stratiform instability
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Question
• Is the intraseasonal signal a moisture mode that differs fundamentally from convectively coupled waves?
Moisture mode: Growth comes from negative effective gross moist stability. The disturbance will propagate through column MSE sources and sinks (e.g. Yu and Neelin, 1994; Sobel et al. 2001; Fuchs and Raymond, 2002, 2005, 2007; Sugiyama, 2009ab)
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Decay time of T, q anomalies of an isolated moist convecting atmosphere column
Kuang, JAS, In press
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Column MSE excess is expressed in terms of a shift in the reference profile of the Betts-Miller scheme
The most slowly decaying eigenmode
Decay time is ~15days, the time for surface flux to remove the column MSE anomaly (radiation is fixed in this case)
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• This slowly decaying mode could provide the memory that reddens the spectrum in frequency.
• The decay time will be modified by diabatic sources, export of column MSE, which could lead to growth. The disturbance will propagate through column MSE sources and sinks (e.g. Yu and Neelin, 1994; Sobel et al. 2001; Fuchs and Raymond, 2002, 2005, 2007; Sugiyama, 2009ab)
• The above moisture modes are either stationary or propagate by horizontal moisture advection
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Is horizontal moisture advection key to the propagation of the intraseasonal signal seen
in the simulation?
• Use zonally rotated u,v fields to compute horizontal moisture advection
u1,v1,q1 u2,v2,q2 u3,v3,q3 u4,v4,q4
longitudenormal
u1+i,v1+i,q1
Rotate wind for q advection
u4+i,v4+i,q4u3+i,v3+i,q3u2+i,v2+i,q2
i is chosen randomly each day
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• This disrupts the coherence between u,v and q thus removes phase dependent horizontal moisture advection on time scales longer than a day
• But preserves the mean horizontal q advection and allows some coherent sub-daily horizontal q advection
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Check climatology is not greatly changed
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Phase dependent horizontal moisture advection is removed
Normalized spectra
Control
0.1
0.4
0.3
0.2
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• Horizontal moisture advection is key to the propagation of the intraseasonal signal but not that of convectively coupled waves
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Zonally homogenize surface heat fluxes
Control
latitude
ControlHomogenized sfc hflx
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Zonally homogenize radiative heating
Control
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Column MSE budgets
Work by Joe Andersen
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Why does radiative feedback weaken convectively coupled Kelvin waves?
Radiative heating anomaly Temperature anomaly
Work by Joe Andersen
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An illustration with column CRM experiments
Anomalous radiative heating applied more in the lower troposphere
No anomalous radiative heating
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Vertical distribution of the anomalous radiativeheating can affect the gross moist stability
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Conclusions
• The intraseasonal signals appear to be moisture modes as they depend on diabatic sources and their propagation depends on horizontal moisture advection
• Effects of radiative feedback on both convectively coupled waves and the intraseasonal variabilities can depend on the vertical structure of the anomalous radiative heating
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Width of the ITCZ
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Width of the ITCZMedium
WiderNarrower
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80km global raveWRF rainfall climatology
Kuang, Walker, Andersen, Boos, Nie, in preparation
Observations
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Rainfall Spectra (γ=1)
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Rainfall Spectra (γ=20)
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Normalized rainfall Spectra (γ=20)
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OLR Spectra (γ=20)
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Normalized OLR Spectra (γ=20)
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Realistic WRF run without sub-seasonal surface flux variations
Kuang, Walker, Andersen, Boos, Nie, in preparation
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Realistic WRF run without sub-seasonal surface flux variations: OLR spectra
Kuang, Walker, Andersen, Boos, Nie, in preparation
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Aquaplanet warm pool
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Aquaplanet warm pool rainfall spectra
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Normalized
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A zonally symmetric case